1. Field of the Invention
The present invention relates to the technology of magnetic resonance imaging (MRI) equipment, and particularly to a method for improving the imaging quality of MRI equipment and MRI equipment.
2. Description of the Prior Art
MRI equipment is used to execute magnetic resonance (MR) sequences for obtaining corresponding images. The magnets of MRI equipment, especially of permanent magnetic MRI equipment, for generating the basic magnetic field are generally made of rare earth materials of high magnetic permeability, such as neodymium, iron, boron, etc. A disadvantage of this type of magnet is its very large temperature coefficient, which is sensitive to temperature changes. When the temperature of the magnets changes, it will cause the homogeneity of the magnetic field within an imaging field of view to deteriorate and thus lead to field drifting; according to the Larmor equation, the field drifting will directly cause frequency drifting, thereby leading to deterioration of the imaging quality. There is thus a need to constrain the influence of relevant factors, so as to improve the imaging quality of the MRI equipment. Two types of technical solutions available in the prior art for improving the imaging quality of MRI equipment are introduced below.
The first technical solution is to improve the imaging quality of the MRI equipment by keeping the magnet temperature constant.
In MRI equipment, the magnet temperature generally needs to be maintained at 30° C. to 32° C., while the common room temperature is 18° C. to 25° C. Therefore, a heating element is conventionally arranged in the MRI equipment for heating the magnet, to ensure that the temperature in the MRI equipment can meet the temperature requirements. However, when the MRI equipment is operated continuously to execute the MRI sequences for a long time (for example, several hours), its gradient system will produce a heating effect, which causes significant temperature increase (i.e., causing substantial temperature drifting) in the magnets. In this case, the magnet temperature needs to be reduced, to ensure that its temperature meets the temperature requirements. For this reason, the first technical solution is directed at keeping the magnet temperature constant by controlling the heating time for which the heating element heats the magnet.
FIG. 1 is a schematic view of the structure of known MRI equipment corresponding to the first technical solution. Referring to FIG. 1, the MRI equipment is composed of upper and lower portions which are symmetrical with respect to a central transverse axis. Except for a temperature control device 7, the remaining construction structures of said upper and lower portions are identical. Specifically, each of the upper and lower portions has a radio-frequency transmitting coil 1, a magnet 2, a gradient coil 3, a temperature sensor 4, a heating element 5, a filter board 6, a heating power supply 8 and a relay output switch 9. In the following description, when it is necessary to distinguish the identical elements of the upper and the lower portions, prefixes “upper” and “lower” will be respectively added for distinguishing them. The relationship between various elements in FIG. 1 will be described below using the upper half of the MRI equipment as an example:
The radio-frequency transmitting coil 1 and the gradient coil 3 are wound respectively on the magnet 2, and the radio-frequency transmitting coil 1 is closer to the central transverse shaft than the gradient coil 3.
The temperature sensor 4 is connected to the magnet 2 for measuring the temperature of the magnet 2, and the measured temperature is used as one of the input signals to the temperature control device 7.
One side of the heating element 5 is connected to the magnet 2, while the other side is connected to the relay output switch 9, for regulating the power output by the magnet 2 according to the control of the relay output switch 9, that is, to heat the magnet 2 with a corresponding output power according to the control of the relay output switch 9.
There are four channels of input signals and two channels of output control signals existing in the temperature control device 7. The four channels of input signals are, respectively, the preset set temperature TS1 of an upper magnet and the set temperature TS2 of a lower magnet, and the temperature Ta1 of the upper magnet measured by the upper temperature sensor 4 and the temperature Ta2 of the lower magnet by the lower temperature sensor 4. The two channels of output signals are respectively, a channel of output control signal obtained according to TS1 and Ta1 for controlling the upper heating power supply 8; and another channel of output control signal obtained according to TS2 and Ta2, for controlling the lower heating power supply 8.
The heating power supply 8 is used to switch on or switch off the power supply of the relay output switch 9 according to the control signal received from the temperature control device 7.
The filter board 6 is located between the left half portion of elements and the right half portion of elements. The left half portion of elements includes the temperature control device 7, the heating power supply 8 and the relay output switch 9, while the right half portion of elements comprises: the radio-frequency transmitting coil 1, the magnet 2, the gradient coil 3, the temperature sensor 4 and the heating element 5.
The working principle of the MRI equipment in FIG. 1 is as follows. The temperature control device 7 measures the temperature of the magnet 2 with the temperature sensor 4 and controls the on-off time of the output relay 9 of the heating power supply 8 according to the difference between the measured temperature and set temperature, or regulates the output duty cycle of the heating power supply 8 according to this difference, so as to regulate the output power of the magnet heating element 5 and to keep the magnet temperature as constant as possible.
The temperature control device 7 generally generates the control signals by a proportional/integral/differential (PID) algorithm, and specifically the above-mentioned process for generating the control signal according to the difference between the measured temperature and the set temperature is as follows. The temperature control device 7 regulates its PID parameter according to the difference between the measured temperature and the set temperature, and generates a corresponding control signal by using the PID algorithm, so as to regulate the output power of the heating element 5.
The abovementioned technical solution is intended to dynamically regulate the power output to the magnet 2 simply in accordance with the temperature variation of the magnet 2. However, since the magnet 2 is a huge thermal sink, it is difficult for the PID parameter of aforementioned technical solution to guarantee the synchronization between the regulation of output power and the temperature variations when MRI scanning sequences are running continuously, phenomena such as hysteresis, advance, oscillation, etc., will inevitably exist. Furthermore, the regulation period of the solution is long, allowing substantial temperature fluctuation to exist in the magnets. Therefore it is difficult to achieve the purpose of improving imaging quality.
The second technical solution is to improve the imaging quality of the MRI equipment by compensating inhomogeneities of the magnetic field that occur.
FIG. 2 is a schematic view of the structure of known MRI equipment corresponding to the currently available second technical solution. Referring to FIG. 2, structurally similar to the MRI equipment shown in FIG. 1, the MRI equipment is composed of an upper portion and a lower portion which are symmetrical with respect to a central transverse axis. The MRI equipment includes a radio-frequency transmitting coil 1, a magnet 2, a gradient coil 3 and a filter board 6, and further has a shim coil 10 and a shim power supply 11. The relationship of various elements shown in FIG. 2 is explained using the upper half of the MRI equipment as an example.
The radio-frequency transmitting coil 1 and the gradient coil 3 are wound respectively on the magnet 2, and the radio-frequency transmitting coil 1 is closer to the central transverse shaft than the gradient coil 3.
The shim coil 10 is wound on the magnet 2, and is positioned between the radio-frequency transmitting coil 1 and the gradient coil 3, and the shim power supply 11 provides power supply to the shim coil 10.
The working principle of the MRI equipment shown in FIG. 2 is as follows. A multi-step (multi-channel) shim coil 10 and a multi-step (multi-channel) shim power supply 11 are designed so as to detect the homogeneity of the basic magnetic field before executing sequence for scanning and the homogeneity of the basic magnetic field is compensated by regulating the current flowing through the shim coil 10. This solution is high in costs and complicated in its regulation, furthermore, it is difficult to dynamically compensate the field drifting generated during the sequence scanning. Therefore it is difficult to achieve the goal of improving imaging quality.
It can be seen from the above description that, in the currently available technical solutions there are respective shortcomings in improving the imaging quality of MRI equipment, and it is difficult for either approach to achieve the goal of improving the imaging quality.